feat: add support for .NET SDK V4

[philasmar]

Description

• Removed the net35 target, updated net45 to net472 and added a net8 target.
• Removed the custom preprocessor directives and used the native ones
• Update the .NET SDK dependencies to the latest version 4.0.0.preview.2
• Deleted parts of the code that still reference AmazonS3EncryptionClient from the SDK, which was deleted.
• Disable obsolete warnings related to AmazonS3EncryptionClient which we agreed not to remove from this version of the library
• Removed blocks of code that are related to net35

Motivation and Context

This change updates the library to use .NET SDK V4 and allows customers that are wanting to upgrade to .NET SDK V4 to continue using this library.

Testing

• Ran the unit tests and integrations tests locally and they passed

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have read the README document
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed

License

• [x] I confirm that this pull request can be released under the Apache 2 license